Study On The Release Characteristics And Mechanism Of Uranium From Granite-type Uranium Tailings

Uranium has both radioactivity and toxicity.With the increase of uranium mining and metallurgy intensity,U is released to environment in a large amount,resulting in a serious ecological threat.Uranium tailings is the source of uranium pollution,studying the release behaviors and mechanisms of U in it,is an important way to reveal the formation mechanism of U pollution.It can also provide scientific basis for U pollution control.In this study,the typical UMTs(granite type)in Guangdong province of China were taken as the research object.Leaching experiment,solid-phase characterization technology,Schultz sequential extraction method and adsorption-desorption experiment of UMTs were combined to reveal the migration and release behavior of U in UMTs and its mechanism.In detail,the release behavior and mechanism of U under different pH and different particle sizes were investigated by using the short-term leaching test method ANS-16.1,which was proposed by the American national standard for the determination of leachability of radioactive wastes.And the control mechanism of U release is also revealed by diffusion model.The solid-phase characterization techniques were used to reveal the changes of U-containing minerals,U-adsorbed minerals and the content of U in that minerals before and after leaching,and the chemical speciation of U in UMTs before and after leaching was analyzed by Schultz sequential extraction method(proposed for radioactive elements such as U).Moreover,the adsorbed U from origin and ground UMTs were extracted by sodium carbonate/sodium bicarbonate solution.The above mentioned methods combined with the results of ANS-16.1 were used to reveal the release mechanism of U.Finally,the effects of the adsorption-desorption process on U migration and release behavior from UMTs(after removing the adsorbed uranium)were studied by adsorption tests,associating the variation characteristics of U species under different conditions calculated by PHREEQC software.The main findings are as follows:(1)The leaching tests of ANS 16.1(pH)show that,UMT_pH=2display the strongest release capacity,following by UMT_pH=3,and there is no linear relationship between the release capacity of UMT_pH=4,5,6and the pH of corresponding leachants.The following predominant factors contribute to the discrepancy of release capacity,a.the release capacity of inactive U varies greatly under different pH leaching agents;b.U mainly exists in the form of carbonate binding/clay adsorption state,and the effects of different pH conditions on carbonate dissolution and U adsorption capacity are different;c.the formation of U-containing secondary minerals(e.g.,uranyl phosphate,schoepite and Na₂U₂O₇).(2)The leaching tests of ANS 16.1(particle size)show that,a.there is no linear relationship between U release capacity and particle size;b.the mainly release mechanism of U can be well described by diffusion model;c.the suspended particles in the leachate carry a considerable amount of U,which is often ignored in similar studies.However,it affects the migration and release mechanism of U to a certain extent;d.the dissolution of calcite,clay minerals and Fe/Mn oxides as well as the oxidation of sulfide play an important role in the release of U.e.U release capacity and mechanism of UMTs with different particle sizes are different,which is mainly due to the difference in mineral composition,resulting in different U forms and tailings charge.(3)The adsorption tests of U on tailings show that:a.the reabsorption of U on UMTs is rapidly and the adsorption-desorption process of U has an important effect on the release of U from UMTs;b.the charge of UMTs and the species of U in leachates are affected by its mineral composition,especially the content of calcite.Furthermore,the adsorption capacity and mechanism of U from UMTs are affected;c.Clay minerals play an important role on the adsorption of U;d.the presence of PO₄^3-can lead to the formation of(UO₂)₂(PO₄)₂4H₂O and hydroxyapatite,which will inhibit the migration of U.The above results reveal the characteristics and mechanism of U release from typical UMTs,which is benefit for the prevention and control of UMTs pollution.